1. Field of the Invention
The present invention relates to a method of measuring a mark position and a measuring apparatus.
2. Description of the Related Art
An exposure apparatus projects a pattern image of a reticle onto a resist coated on a wafer thereby exposing the resist. To achieve high accuracy of the position where the reticle pattern is projected, it is important to achieve high accuracy in relative alignment between the reticle and the wafer.
A mark (alignment mark) used in the alignment is formed together with the reticle pattern on the wafer by an expose-and-transfer process. A plurality of alignment marks located in predetermined shots are detected using an optical system, and an array of all shots is calculated based on positions of the detected marks, and alignment is performed based on a calculation result.
The alignment marks can vary in shape among wafers and among shots. This can cause a position detection error (hereinafter also called a WIS (Wafer Induced Shift)) due to a variation in a wafer process condition to occur in a mark position detection process, which in turn can cause a reduction in alignment accuracy. Japanese Patent Laid-Open No. 2004-117030 discloses a technique of reducing the WIS by performing offset calibration. The offset calibration is a process of calculating the shift of the position of the alignment mark actually detected by the detection system from the true position and correcting the detected position based on the calculated shift.
Japanese Patent Laid-Open No. 2004-281904 discloses a technique of correcting the mark position detection. In this technique, a transfer characteristic of an optical system is determined in advance, and image data of an alignment mark is corrected (reconstructed) using an inverse function of the transfer characteristic of the optical system. Note that an error caused by an apparatus (tool induced shift (Tool Induced Shift)), which will be described later, is one of errors caused by the transfer characteristic of the optical system.
The WIS is not the only cause of the position detection error, but there can be other causes. An example is an error produced by an exposure apparatus (mark detection optical system). Such an error is generically called a tool induced shift (TIS). Another example is an error produced by an interaction between TIS and WIS (TIS-WIS interaction), which can cause a reduction in alignment accuracy. Factors that can cause the WIS include a height of a step (a depth of a recess) of the alignment mark, asymmetricity of the alignment mark, and nonuniformity of resist coated on a wafer. Aberration of the mark detection optical system can be a factor of the TIS.
It is difficult to eliminate TIS perfectly. Therefore, when there is a WIS (such as an error caused by a low step height of a mark, nonuniformity of a resist coated on a wafer, etc.), a TIS-WIS interaction can make it difficult to achieve high accuracy in detection of an alignment mark position. Even when the same optical system is used, the presence of a TIS can cause a greater position detection error to occur for an alignment mark with a smaller step height than for an alignment mark with a larger step height.
In the technique disclosed in Japanese Patent Laid-Open No. 2004-281904, it is assumed that the transfer characteristic of the optical system is used in the correction. However, in practice, it is difficult to accurately measure the transfer characteristic of the optical system, and a measurement error can cause a reduction in accuracy in reconstruction of image data of the alignment mark. Besides, even if the influence of the TIS can be reduced, this does not necessarily result in a reduction in influences of variations in wafer process conditions such as a variation in the step height of the alignment mark, a variation in resist thickness, etc.
In view of the above, the present invention provides a method and an apparatus for measuring a mark position with a less measurement error caused by variations in wafer process conditions.